1. Field of the Invention
The present invention relates to an electroluminescent phosphor and an electroluminescent element using the same.
2. Description of the Related Art
An electroluminescent element is prepared in the following way. A phosphor layer has an electroluminescent phosphor dispersed in a dielectric material. On both surfaces of the phosphor layer, electrodes are disposed. At least one of them is a transparent electrode. An alternating voltage is applied between the electrodes to make the electroluminescent element luminesce. Electroluminescent elements are mainly used in display devices or backlighting of display devices.
An electroluminescent phosphor for use with electroluminescent elements generally comprises zinc sulfide as a phosphor base material, at least one kind of copper and manganese as an activator, and at least one kind of chlorine, bromine, iodine and aluminum as a coactivator.
An electroluminescent phosphor based on zinc sulfide is prepared in the following way, for instance. First, zinc sulfide as a base material is mixed with an activator source and a coactivator source. Further, magnesium chloride, barium chloride, sodium chloride or the like is added thereto as a crystal growth agent, followed by thorough mixing. This mixture is fired at a temperature in the range of from 1000 to 1300.degree. C. to form an intermediate. The intermediate, after being crushed, is heated at a temperature in the range of from 600 to 1000.degree. C. Thereby, a zinc sulfide system electroluminescent phosphor including an activator and a coactivator is obtained (cf. U.S. Pat. No. 2,957,830). The added crystal growth agent is removed by washing the fired body with water (cf. U.S. Pat. No. 4,859,361).
The existing zinc sulfide system electroluminescent phosphor can not necessarily satisfy characteristics demanded for display devices such as brightness and life. That is, there are problems to be improved. Accordingly, so far a variety of kinds of improvements have been applied to zinc sulfide system electroluminescent phosphors.
For instance, zinc sulfide is known to take a crystallographic structure of hexagonal system at a temperature of 1020.degree. C. or more and that of cubic system at a temperature below that. Luminescent efficiency due to electroluminescence is higher for the cubic system. However, rather than getting the cubic system from the beginning, by transforming to the cubic system through the hexagonal system, the luminescent efficiency can be made higher.
Japanese Patent Laid-Open Publication (KOKAI) No. Sho 61-296085 discloses a method of transforming the crystal system of zinc sulfide system electroluminescent phosphor. According to this method, to zinc sulfide mixed with an activator a compound of alkali metal element and a compound of alkaline earth metal element are added to form an intermediate of hexagonal system. Thereafter, by applying high pressure, a zinc sulfide system electroluminescent phosphor of cubic system is obtained.
Japanese Patent Laid-Open Publication (KOKAI) No. Sho 57-145174 discloses a method of improving efficiency and life of zinc sulfide system electroluminescent phosphors. In this method, the efficiency and life of zinc sulfide system electroluminescent phosphors are improved by optimizing the addition amount of the aforementioned activator and coactivator.
As described above, various kinds of methods of improving the luminescent efficiency and life of the existing zinc sulfide system electroluminescent phosphors have been proposed. However, the existing methods are gradually becoming incapable of satisfying the present requirement level for brightness and life. In particular, recent display devices are demanded to improve further the brightness and life. Accordingly, electroluminescent phosphors being used with them are demanded to improve further the brightness and life.
U.S. Pat. No. 2,952,642 discloses a zinc sulfide system electroluminescent phosphor that includes lead and copper as activator, and 0.001% of magnesium. However, the zinc sulfide phosphor including lead is still poor in emission efficiency and life characteristic, accordingly it can not satisfy performance that the market demands.